1. Field of the Invention
The invention relates to optical filtering elements and is more particularly concerned with optical lens elements including transparent fluorescent materials, which elements absorb visible radiation of certain wavelengths and emit and transmit radiation at longer, but also visible, wavelengths without reflecting substantial amounts of radiation.
2. Description of the Prior Art
Ordinary radiant energy, such as that provided by the sun, is normally grouped into three regions, the near ultraviolet (about 300-400 nm), the visible (about 400-700 nm) and the near infrared (about 700-4000 nm). Many types of optical filtering elements are provided for various purposes, but by far, the most common type of optical filtering element used with these wavelengths is that utilized in a sunglass lens. An important requirement for sunglass lenses is that they absorb significant amounts of visible radiation. Natural daylight or sunlight includes a nearly uniform distribution of light energy over the visible region. Absorption of or reduction in the transmission of visible light has been achieved, for example, by the use of lens systems employing various dyes, opacifiers (such as dyes and thin metal coatings) and/or polarizing materials, either alone or in various combinations. Dyes which absorb transversing light well in the visible portion of the spectrum have been used in optical systems, including sunglass lenses. The primary criterion for selection of such dyes has been their ability to absorb broad bands of radiation energy with minimal alteration of the spectral distribution of the radiation. However, the desired decrease of light intensity which is provided by dyes which produce broad band visible radiation absorption can have the detrimental effect of lessening color perception and visual acuity. Such prior art dyes have not been known to be capable of providing multiple band re-emission or fluorescence of the absorbed radiation which is why they lessen color perception.
Certain optical systems utilizing fluorescent dyes are known. So-called "aura" glasses have utilized unstable spectral sensitizing fluorescent material, such as pinacyanol bromide, but have no practical visual utility as sunglasses as the fluorescent material is unstable and the thickness of the fluorescent layer is such that light falling on them is reflected as fluorescent radiation. In laser systems, certain fluorescent dyes in a liquid solution or carrier, have been utilized in optical elements to produce lasing effects. Such dye systems are required to be liquid in order to be pumped or replenished. Other laser systems utilizing fluorescent dyes in solid optical elements have been provided as a part of a waveguide structure to receive lateral, as opposed to transversing light.
Dyes which absorb visible radiation and fluoresce in the visible spectrum are generally chemically unstable and subject to photodegradation, for example, in sunlight. Therefore, such fluorescent dyes have not normally been thought of as being suitable for use as energy absorbing dyes in sunglasses. Additionally, the utility of many fluorescent dyes, whether stable or not, is diminished, in a practical sense, by their transmission characteristics in the visible region. Only a limited number of fluorescent dyes or combinations of fluorescent dyes, display the broad absorption and emission characteristics needed for a useful filtering lens in the visible radiation region.
It has now been determined, experimentally, that even when stable fluorescent dyes are used in sunglasses, that they surprisingly and unexpectedly are unsatisfactory for normal human wear and use because they reflect a distasteful fluorescent glow, often in a harsh red color.
Also in the prior art, U.S. Pat. No. 3,426,212 discloses systems for substantially modifying the wavelengths of radiation, consisting of at least two layers, each layer containing fluorescent substances which fluoresce at different wavelengths, so that light transmitted and fluoresced to a different wavelength by one layer impinges on the second layer which further fluoresces and further modifies the wavelength, the two wavelength modifications generally being in the same direction. The systems disclosed by this reference are not intended for human eye wear and indicate no knowledge of or concern for fluorescent glare or critical minimum thicknesses to avoid such glare.
U.S. Pat. No. 3,214,382 discloses an optical system including phosphorescent material, but does not teach the use of fluorescent material in such a system.
U.S. Pat. No. 2,113,973 discloses a fluorescent layer including unstable rhodamine B which is somewhat protected from decomposition by use of a light filter which absorbs much of the radiation which causes decomposition. The systems taught by this reference are intended only for fluorescent reflection and not for light transmission. The reference indicates no intended use for human eye wear and indicates no knowledge of or concern for fluorescent glare or critical minimum thickness to avoid such glare.
Of other interest, although non-anticipatory of the present invention, are U.S. Pat. Nos. 2,498,593; 2,386,855 and 2,851,423, Canadian Pat. No. 504,529; United Kingdom Pat. No. 569,920; Shah, J. et. al, "Excited-State Absorption Spectrum of Cresyl Violate Perchlorate," Appl. Phys. Lett., 24(11), pp. 562-564 (June, 1974) and Chem. Abstracts, 85:114369n, Drexhage, K. et. al, "New Highly Efficient Laser Dyes."